1. Field of the Invention
The present invention relates to an electrophotographic toner containing a binder resin and a magnetic substance and also relates to a method for forming an image comprising a latent image forming step for forming a latent image on a latent image carrying substrate, a development step for forming a toner image by developing the latent image by making use of a developer on a developer carrying substrate, a transfer step for transferring the formed toner image to a transfer medium and a fixation step for fixing the toner image on the transfer medium.
2. Description of Related Art
In recent years, a digitalization has been developed in not only a printer but also a copying machine or a plotter for a large-sized drawing and a latent image of high resolution such as delicate gradation by small Chinese character (Kanji) or dot has been able to be formed. Then, studies have been carried out to discover a toner which can be used to accurately develop latent images of high resolution.
On the other hand, a two component development method using a toner and carrier such as iron powder etc., and a magnetic single component development method using only a toner containing a magnetic substance are known as dry development methods in various kinds of electrostatic copy systems currently practically used. Further, in the development method using a magnetic single component toner, a developing machine may be simplified because there is no need to use an automatic density controller or the like which are required for the two component development method, and maintenance such as replacing the carrier or the like is not needed because there is no carrier. Therefore, a magnetic single component toner which is able to develop accurately the latent image of high resolution has been desired.
It is, however, difficult for the magnetic single component toner to develop accurately the latent image of high resolution, since the magnetic single component toner tends to aggregate and to form clumps due to magnetism of magnetic substance compared with non-magnetic toner, used for two component development, free from the magnetic substance.
Furthermore, the magnetic single component toner has an inferior fixing property compared with a non-magnetic toner used for two component development, since the magnetic single component toner contains a magnetic substance and the proportion of binder resin in the toner decreases.
In order to satisfy the strong requirement for a small-sized machine and economy in energy, a magnetic single component toner which is able to fix in lower energy has been desired, and various kinds of binder resins have been studied. Above all a polyester resin has been remarkably studied, since the polyester resin has an ester group contained in a main chain which group is freely rotatable, and has an excellent flexibility and an excellent fixing property at low temperature as described in, for example, Japanese Parent Application Publication (JP-B) Nos. 46-12680 and 52-25420.
However, when the magnetic single component toner containing the polyester resin disclosed in these Publications is used in heat fixing system in which heat and pressure are applied by means of two rolls, the toner adheres to the heat rolls, which causes offset which stains the next copy. For example, when postcards are continuously printed, the heat of a portion of the heat roll surface, with which portion each of the postcards is in contact, is absorbed by the postcards. Accordingly, the whole heat roll is heated to maintain constant the temperature of this portion of the heat roll surface contacting the postcards. As a result, the temperature of the other portion of the heat roll surface becomes higher than that of the portion with which each of the postcards is in contact. Then, when a paper of a large size is printed immediately after printing of the postcards, the toner on the paper which toner is in contact with the other portion of the heat roll surface (i.e., the portion other than the portion contacted by each of the postcards) causes offset.
In order to improve the fixing property of a toner containing a polyester at high temperatures and prevent the offset thereof as above described, there been proposed a magnetic single component toner containing non-linear polyester resin formed by making use of polyol or polycarboxylic acid which have more than or equal to three functional groups in a molecule, or cross linking agent (Japanese Patent Application Laid-Open (JP-A) Nos. 3-11364 and 5-134454).
In these magnetic single component toner, however, while an anti-offset property can be improved the fixing property at low temperature deteriorates.
Furthermore, in order to obtain both a satisfactory anti-offset property and a satisfactory fixing property at low temperatures, a magnetic single component toner containing linear polyester resin and non-linear polyester resin has been proposed. (Japanese Patent Application Laid-Open (JP-A) Nos. 60-4947 and 60-90344)
However, linear polyester resin and non-linear polyester resin can not be homogeneously mixed at the time of milling because of the difference in visocoelastic behaviors thereof. Therefore, two kinds of toners, one kind containing mainly linear polyester and the other kind containing mainly non-linear polyester, are formed by way of a grinding step and a classification step. As a result the toner containing mainly linear polyester adheres to the heat roll at the fixation step, which causes an offset.